In the wake of January's Indian peasant uprising in the southern state of Chiapas, the murder of the country's most-likely next president threatens the entire Mexican political system.
Colosio was gunned down as he campaigned in Tijuana, on the US border. The US now faces turmoil across the Rio Grande.
'Now we face the possibility of having a kind of cadaverous Nafta,' said Larry Birns of the private Council on Hemispheric Affairs. 'If Mexico is not considered a reliable investment environment, then investors are going to say, 'Why go to Mexico? We go to Bangladesh'.'
Mexico has been plunged into its worst crisis since the 1920s, the turbulent period after the 1910-17 revolution which led to the foundation in 1929 of the National Revolutionary Party, later to become the Institutional Revolutionary Party (PRI) which has ruled ever since.
Whether August's six-yearly presidential elections will be held on schedule must now be in doubt. All opposition parties have ceased campaigning out of respect for the murdered politician. A question mark also hangs over the attitude of a military still smarting from President Carlos Salinas de Gortari's order to hold back from crushing the Zapatista National Liberation Army (EZLN) in the south.
A 23-year-old mechanic called Mario Aburto Martinez allegedly shot Colosio. It is said that Mr Martinez shouted 'I saved Mexico', as he was beaten and detained by an angry crowd. Some officials said Mr Martinez was 'crazy' and that it was 'an isolated incident'. Few in Mexico are likely to believe this. There have been too many coincidences recently.
Those under suspicion included the EZLN or its northern sympath isers; Colosio's former rival for the PRI candidacy, Manuel Camacho Solis, or his sympathisers; old- guard PRI factions unhappy with Colosio's plans to continue Mr Salinas's reforms; the military, worried that the PRI's slipping popularity would lead to a left-wing, populist president; the opposition National Action Party (PAN), strong in Tijuana and across the north; the main opposition populist Party of the Democratic Revolution (PRD) of part-Indian Cuauhtemoc Cardenas; drugs mafias wishing to create unrest; even the Catholic Church, which was strongly identified with the Chiapas Zapatistas and blamed by many for backing the rebellion.
A stunned President Salinas, who handpicked his friend Colosio as the PRI's presidential candidate, moved quickly to quell rumours of a state of emergency. 'We will keep in place our regime of liberty and of constitutional order,' he said in a nation-wide television broadcast.
But liberty in Mexico is relative. In a country of more than 80 million, some 90 per cent of the country's wealth is in the hands of a few hundred thousand. Despite Mr Salinas' promises of greater democracy, little had changed until the EZLN, under 'Subcomandante Marcos', jolted the system with its armed rebellion.
Nothing in Mexico has been quite the same since. Even before Colosio's murder, things were threatening to change rapidly. Manuel Camacho Solis, a PRI militant and close friend of Mr Salinas, was openly upset at being passed over as the party's presidential candidate last November. Mr Salinas quickly named Mr Camacho Foreign Minister as a sweetener.
After the Zapatista uprising, Mr Camacho was pulled out of that job and sent to Chiapas to negotiate with the guerrillas. His efforts eclipsed the opening of Colosio's campaign, whose first speech was relegated to a few paragraphs in a press which is largely beholden to the PRI.
Mr Camacho was daily front-page news, smiling and posing with Sub comandante Marcos and the church mediator Samuel Cruz, Bishop of the Chiapas town of San Cristobal de las Casas. Mr Camacho appeared to grant the Zapatistas almost everything they had demanded. They are currently poring over the peace deal but Subcomandante Marcos recently said that they would not lay down their arms until after the elections. Mr Camacho was suspected of electioneering and hinted he might run for president. Then, only a day before Colosio was shot, Mr Camacho said he would not run. 'I want to be president of Mexico, but not at any cost,' he said.
It is not clear how Mr Salinas or the PRI will pick a new candidate.
Leading article, page 17
Obituary, page 30
(Photograph and map omitted)